Conductors (i.e. cables) are one method commonly used to convey electronic video signals from a source device (e.g., a video camera or a DVD player) to a destination device (e.g., a video display screen). Two types of cable commonly used for video transmission are coaxial cable and twisted pair cable. It is desirable for the video signal at the destination device to correspond accurately to the original video signal transmitted by the source device. “Insertion loss” is a term used to describe signal degradation that occurs when a video or other signal is transmitted over a transmission medium such as a cable.
Typically, insertion loss is a function of the cable length: longer length transmission cables will exhibit greater loss than shorter length cables. Coaxial cables typically exhibit less insertion loss than twisted pair cables. However, coaxial cables are more expensive and difficult to install than twisted pair cables. Twisted pair cables typically are manufactured as bundles of several twisted pairs. For example, a common form of twisted pair cable known as “Category 5” or “CAT5” cable comprises four separate twisted pairs encased in a single cable. CAT5 cable is typically terminated with an eight-pin RJ45 connector.
Insertion loss is typically caused by the physical characteristics of the transmission cable. Insertion loss includes resistive losses (also sometimes referred to as DC losses) as well as inductive, capacitive and skin effect losses (also sometimes referred to as AC losses). The AC insertion loss exhibited by a cable is frequency dependent. For example, the insertion loss for a 1500 foot length of CAT5 cable as a function of frequency is shown in FIG. 11. In the example of FIG. 11, the insertion loss generally increases with increasing frequency, with the insertion loss for high frequency signals being significantly greater (−70 dB at 50 MHz) than the DC insertion loss (−2.6 dB at 0 Hz).
Video transmitter/receiver systems exist that amplify video signals transmitted over twisted-pair cables. In such systems, a transmitter amplifies the video source signal prior to being transmitted over twisted pair cable, and a receiver amplifies the received signal. These transmitter/receiver systems allow longer transmission distances over twisted-pair cable than are possible for unamplified signals. However, to prevent signal distortion, the amount of gain (amplification) supplied by the transmitter and receiver must be properly matched to the amount of insertion loss that occurs in the length of the twisted-pair cable over which the video signal is transmitted. If the gain applied is too high, clipping will occur. If the gain is too low, low-level portions of the original input signal may be lost. Ideally the system gain should be flat across the frequency spectrum. High frequency loss results in smearing and loss of focus in the video.
Depending on the length of cable over which the signal is transmitted, the amount of amplification required to compensate for insertion losses may be substantial. When substantial amplification is provided to a signal, noise becomes an issue. There exists a need for a video transmission system that automatically compensates for noise incident with the amplification of video signals transmitted over appreciable distances via conductors, including twisted pair cables.